What’s a Watt?
To understand and appreciate solar, you first should understand some basic concepts of electricity, and how electricity is billed to you.
All electrical appliances tell you the number of amps, volts, and watts that they use. Their relationship is simple: Watts = Volts * Amps (W = V *A). Many battery systems run on 12V systems, so for example, a 12V cell phone charger might be 0.9 amps, or, 12 * 0.9 = 10.8 watts. That’s about as much electricity as can be pumped into a smartphone battery at one time.
Most devices in your home consume a lot more power than the smartphone, however. Your home (if it’s in the US) is wired at 120V Alternating Current (AC) and some appliances (dryers, water pumps) are wired in at 240V. A washing machine may have a peak load of around 12 amps at 240 volts, or, close to 2,880 watts. That’s as much current as can charge 160 smartphones!
1,000 watts = 1 kilowatt. However, you are billed for electricity in kilowatt-hours, the product of a certain amount of electricity times a period of time. So for instance, that washing machine, pulling 2,880 watts, for 5 minutes, would consume 240 watt-hours (0.24 kilowatt-hours) of electricity. At today’s rate (around 15.5 cents) that amount of electricity would cost you about 4 cents.
The accumulation of ALL of the electric loads in your household, times the amount of time that you run them, culminates in your total electric bill from the utility. The energy loads that end up big hogs can be deceiving. For example, a 100-watt incandescent lightbulb, left on 24 hours a day, 365 days a year, will consume 876 kilowatt-hours each year, at a cost of over $130!
How Solar Fits In
Solar panels are the opposite of electric loads, they generate a certain amount of electricity for every minute they are exposed to sunshine. Solar panels are rated in watts (typically 245 to 280 watts) and collections of them for homes are rated in kilowatts. For example, 20 solar electric panels rated at 250 watts results in a 5 kilowatt solar electric array.
An oversimplification is to say that a 5 kilowatt solar electric system will generate 5 kilowatt-hours (kWh) for each hour that they are exposed to the sun. Realistically, some amount of the solar energy is lost in the wiring process and slight inefficiencies in the equipment that converts direct current (DC) electricity generated by sunshine into 120V AC electricity consumed in your home. Also, the sun is rarely constant for a full hour, any clouds or changes in sun intensity will affect the real-time performance of a solar array.
So to predict how much electricity a 5 kilowatt solar array will generate, we take data on regional solar insolation (the amount of sun expected to strike the earth’s surface in our region, each day of the year) and build out a model of expected solar generation. The National Renewable Energy Labs (NREL) has an excellent calculator, PV Watts, which uses 25-years of weather data to assess expected solar insolation for a location.
A full analysis of a solar array’s expected production based on climate, adjusted for the angle of the solar array, and orientation (azimuth) towards the sun, and adjusted again for any shading, results in a prediction of a system’s output over the course of the year. As a rule of thumb, each 1 kilowatt of installed (nameplate) grid tied solar, on a good site, with generate around 1,250 kWh/year in New England. A typical 5kw array for a home will generate about 6,250 kWh/year.
Energy efficiency enthusiasts make the claim that the easiest kilowatt-hour to create, is the one you don’t have to use, hence, “Negawatts!”
When we start designing a solar electric array for a system, our first step is to figure out exactly how much electricity they are using, and how much solar resource is available at their location. A homeowner with a very tight roof, who can only fit 16 panels, for instance, has a different set of decisions to make than a homeowner who has a vast south-facing barn where they could potentially install much more solar electric generation than they actually need.
Most of our customers see solar as part of a road towards energy independence, and taking reasonable aims to reduce energy consumption in tandem with a solar installation is a highly recommended strategy. It can work out to be much more expensive to produce electricity that is used relatively inefficiently – for example, powering old refrigerators and water heaters. Generally, it is cost-effective to make some (not all) energy efficiency upgrades as a path towards meeting all of your needs with solar electricity.
However, there are also “good” reasons to increase your electric bill! With heat pumps, electric cars, and electric water heating solutions available, a home can essentially eliminate oil, propane, or natural gas completely and be fully electric powered. Many of our customers add enough solar to meet all of their needs initially, then upgrade the system over time to supply power for a heat pump or electric car.
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